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The microtremor method (MTM) using spatial autocorrelation (SPAC) processing is a useful tool for gaining thickness and shear wave velocity (SWV) of sediments. These parameters are essential for site response modeling and regolith classification for earthquake hazard and risk assessments. Complications arise if the wave field is strongly directional (ie. insufficiently averaged in azimuth) or contains multiple Rayleigh modes. Theoretical studies compare the use of triangular, hexagonal and semi-circular arrays and show how the latter is preferable in the presence of strongly directional seismic noise, while the hexagonal array is superior in maximising the range of detectable wavelengths. Both hexagonal and semicircular arrays can identify multiple-mode wave propagation.

Observed coherency-frequency curves are inverted in coherency space (without the intermediate step of obtaining a dispersion curve from field data) to yield a profile of SWV and layer thicknesses to depths up to one hundred metres. The MTM using SPAC has achieved a precision of+-10% or better in the Vs30 zone of unconsolidated but moderately homogeneous sediments. The results show a good to strong correlation with seismic cone penetrometer tests (SCPTs) from Perth (Western Australia). Moreover, the microtremor data has an additional advantage of being capable of non-invasively detecting the base of sediments where the SCPT method fails in coarse gravels.